Various harmful electron waves and electromagnetic waves generated in circuits of electronic instruments may disturb the function of peripheral electronic parts or devices, deteriorate performance, produce noise, degrade images, decrease their service life, and thus result in production of inferior products. In order to protect sensitive electronic equipment from such electron waves and electromagnetic waves, various electron wave- and electromagnetic wave-shielding materials have been developed. For example, a variety of metal plates, metal coated fabrics, conductive paints, conductive tapes or conductivity-imparted polymer elastomers have been suggested.
In general, the following methods have been used in order to impart conductivity to polymer elastomer resins.
For example, during the production of polymer elastomer resins, finely divided conductive powder or filler such as common carbon black, graphite, silver, copper, nickel or aluminum is dispersed uniformly in the resins. In order to impart conductivity to the polymer elastomer resins, it is necessary to form a pathway of interconnecting filler particles in the polymer resin. In other words, metal particles or carbon black particles should be in a close contact state so that the conductive particles permit passage of electrons.
For example, when carbon black particles are blended with urethane resins to impart conductivity in some applications, 15-30 wt % of carbon black particles are used based on the weight of the resins. However, 40 wt % or more of carbon black particles may be desired in order to obtain improved conductivity. Introduction of carbon black particles in such a large amount makes it difficult to disperse the particles uniformly, and reduces melt viscoelasticity of resins, resulting in agglomeration of filler particles and a significant increase in viscosity. When metal powder is used, it is blended with resins in an amount of 2-3 times by weight of the corresponding amount of carbon black in order to obtain electroconductivity. In this case, dispersibility becomes poor and specific gravity increases.
In brief, according to the above-described method of introducing conductive filler to polymer resins, it has been difficult to obtain polymer resins exhibiting acceptable electroconductivity as well as impact- and vibration-protecting properties.
According to another conventional method, electron wave- and electromagnetic wave-shielding material is obtained by coating various fabrics, non-woven webs, paper or other plastic films with a coating agent comprising a conductive agent. Such materials include metal plated fabrics and conductive tapes. However, because such materials lack volumetric conductivity, they are merely used in applications requiring surface conductivity.
Further, according to still another conventional method, in order to impart a silicone sheet with conductivity, an excessive amount (70 wt % or more) of filler is used in the silicone sheet. However, the method is not cost-efficient due to the high cost needed for such a large amount of filler.
Particular examples of approaches to imparting conductivity to polymer resins or elastomers include: Japanese Laid-Open Patent No. Hei 9-000816; Japanese Laid-Open Patent No. 2000-077891; U.S. Pat. Nos. 6,768,524; 6,784,363; and 4,548,862.
However, the above-mentioned methods according to the prior art have disadvantages in that they need a separate step of adhesive-treatment or they have to use additional adhesives for example, double-sided adhesive tapes.